The Earth is not round, but the Sun — contrary to long-standing scientific belief — is a different story.

To be clear: the Earth certainly isn't flat, but it's definitely not a perfect sphere. Technically speaking, it's an oblate spheroid — a sphere that's been squashed, such that the distance from Earth's center to sea level is about 13 miles greater at the equator than at its poles. Earth isn't really smooth, either; in actuality, it's kind of bumpy.

But the Sun, it so happens, is neither of these things. In fact, according to research published in the latest issue of Science, the Sun is actually the most perfectly round natural object in the known Universe. That's pretty big news, especially because (prior to the publication of this study) astronomers had presumed that the shape of the Sun was changing all the time, squishing and un-squishing into and out of a spherical shape throughout its 11-year solar cycles. But the shape of the Sun, note the researchers in the paper recounting their findings, is actually "remarkably constant."

As a spinning ball of gas, astronomers had always expected our nearest star to bulge slightly at its equator, making it very slightly flying-saucer shaped. The planet Jupiter demonstrates this effect well. Its high rate of spin - once every 10 hours - means that it is almost 7% wider across its equator than the distance from pole to pole.

Now a team led by the University of Hawaii's Dr Jeffrey Kuhn have made the first precise measurement of the sun's equatorial bulge, or its "oblateness". The results were a big surprise. "We were shocked," says Kuhn. The sun doesn't bulge much at all. It is 1.4m kilometres across, but the difference between its diameter at the equator and between the poles is only 10 kilometres.

Scaled to the size of a beachball, that difference is less than the width of a human hair. Only an artificial sphere of silicon that was created as a standard for weights is known to be more perfectly spherical.

Kuhn say these findings would have been impossible without NASA's Solar Dynamics Observatory, which has only been orbiting Earth since 2010. For half a century, attempts to precisely measure the shape of the Sun have been hobbled by the blurring effect of Earth's atmosphere. "Finally, from space," Kuhn says, "we have it nailed."

The researchers' findings are published in the latest issue of Science.